1. Field of the Invention
The present invention relates to a dye-sensitized solar cell that converts solar energy to electric energy, and more particularly, to dye-sensitized solar cells that include ultra-fine semiconductor fibers sensitized with more than one light absorption materials.
2. Description of the Related Art
Dye-sensitized solar cell (DSSC) is a non-conventional photovoltaic technology that attracted much attention due to its cost-effectiveness in harvesting solar energy with appealing properties such as flexibility, transparency, and adaptability in large-area devices. The operating principle of DSSC is illustrated in FIG. 12. Upon illumination, the dyes adsorbed onto the metal oxide semiconductor (usually TiO2) are sensitized to the exited state (S*) by light absorption right at the interface and they dissociate readily to create an electron-hole pair, with electrons subsequently injected into the conduction band of the semiconductor while the holes, at least initially, remain on the sensitizers. The dye ground state (S) is then regenerated by electron donation from the redox system to the oxidized state of the sensitizer (S+). The recuperation of redox system is realized by transporting holes to the counter electrode either in diffusion or hopping mechanism (depending on the transporting mediator). The whole process is finally completed by electron migration via the outer circuit and the device generates electric power from light without chemical transformation.
For decades, DSSC have become one of the most efficient and stable excitonic solar cells. A central feature of this device is utilizing photosensitizing dye that harvests light and generates excitons. In order to achieve high power conversion efficiency based on I−/I3− redox couple system which would be competitive with conventional silicon solar cells, DSSC must absorb as much as 80% of solar spectrum with wave length between 350 and 900 nm. While the traditional ruthenium-based dyes exhibit relative broad adsorption spectrum, it has difficulty in further improving its Power Conversion Efficiency (PCE) due to its low molar extinction coefficients.
Organic dyes, such as metallophthalocyanines (MPcs), shows higher molar extinction coefficient (100,000M−1 cm−1), however, they have narrow absorption bandwidth. Complementally, dye cocktails or co-sensitization has been proposed to enhance the light absorption and extend the absorption spectrum. However, it has achieved only limited success to-date. This is probably due to (i) inferior injection efficiency caused by intermolecular interactions between dyes; (ii) confined surface areas of the photoanode for dyes to be absorbed. Considerable efforts have been made to solve these problems, one option is to separate the adsorption sites on TiO2, which means achieving the proper position of each dye on the desired site, however, there is difficulty in realizing such a concept.
Recently, there have been some efforts on the use of Förster resonance energy transfer (FRET) in DSSC to enhance the light harvesting where an unattached, highly luminescent donor dye was inside the electrolyte to absorb high energy photons and efficiently transfer the energy to the anchored near-infrared acceptor dye. Unfortunately, I3− in the electrolyte was found to partially quench the fluorescence of the donors, therefore only limited improvement in device performance can be achieved with such approach.
In view of the deficiencies of the conventional dye-sensitized solar cells, there is an increasing demand for high efficiency solar cells that are capable of harvesting a broader range of solar energy with improved power conversion efficiency.